I've never had to downgrade Firefox, most of the time I don't even notice its been updated unless it pops up a "Welcome to Firefox vSomething" page which it hardly ever does.

I did try out Zen, and its certainly an interesting browser, but there's a couple of issues with it that I have:

• It's a browser that is as far as I understand, maintained by one person - I learned a while ago not to use critical software that is only maintained by one person
• It's still in alpha, and things change very quickly with it. Unlike Firefox I do notice when Zen updates because some behavior will have heavily changed. A few weeks ago there was an update for example that completely removes the new tab page (pressing Ctrl + T or the new tab icon just opens the URL bar and then when you submit your address/query then it navigates directly to it in a new tab). Interesting change for sure, I'm not sure I dislike it but that is a major behavior to change in an update with no prompt about it being updated, and it wasn't togglable in the settings menu - you had to go to about:config to change it back. When I looked at the GitHub issue regarding this, the dev seemed a bit... unhappy with people's reaction, to put it lightly.
• Because its still in alpha, there can be stability issues like you mentioned; I recently finally switched back to Firefox when I noticed for some reason Zen was causing my GPU to run in high power mode and using high utilization as if I were running a game and dragging the rest of my system down. Thought maybe I'd left a video running in a tab somewhere and that it was just HW accelerated decoding, but nope.

Obviously those last two points as mentioned are more understandable because the browser is in an alpha stage, but browsers are for better or worse very critical pieces of software. I can't have it just randomly crashing out on me, or behavior changing out from under me every week. This combined with people selling Zen as if its the next coming of Christ has kinda left a bad taste in my mouth for it. Don't get me wrong, what the dev has pulled off is incredibly impressive and major props to them for it, but I'll be waiting for it to leave the alpha stage until I'm able to daily drive it. For now I'm just back on regular mainline Firefox.